18 Sol-Gel Applications Flashcards
Applications of sol-gel materials
– Biosensors’ supports
– Precursors for the synthesis of composites
with controlled chemical doping
– Preparation of synthetic opals
– Synthesis of porous materials
Inorganic-organic hybrid material
Sol-gel reactions using more than one precursor
can yield “hybrid” materials. Low-temperature
conditions and availability of precursors makes
sol-gel ideal for synthesizing materials with
unique properties.
Embedding of organic molecules
Dissolve organic molecules in the sol-gel
reaction mixture. Once a gel is formed, the
organic molecules are trapped in the gel matrix.
Polymers, dyes, catalysts can be embedded into
the gel matrix.
Biosensing
biological species embedded in a porous solid
matrix acts as a biosensor for specific molecules.
Biomolecules are sensitive to pH, temperature
changes, and different solvents. Mild conditions
used in sol-gel reactions are ideal for synthesis
of biosensors.
incorporation of organic groups via covalent bonding
Use organotrialkoxysilane and organodialkoxysilane or
more complicated organosilanes for sol-gel processing
(see examples below). Si-C bonds are hydrolytically
stable and are maintained in the final product.
Porous materials
Also known as cellular solids. Used for
insulation, shock absorption, catalysis. A porous
material must have cavities with a large aspect
ratio
Pore type
Closed pores
Open, blind
Open, through
Open, through
Classification of pores
Mobility of molecules within a pore depends on
pore size.
Macropores: Mobility depends on bulk diffusion.
Micropores: Mobility is controlled by
intermolecular interactions between the porous
material and solution.
Templating using molecules
Sol-gel synthesis can be used to prepare porous
materials such as zeolites or mesoporous
silicates. The zeolite or silicate grows around a
templating agent producing a porous structure
displaying long-range order
Adsorption Isotherms
Measuring surface area
fraction of molecules adsorbed
to a surface as pressure increases (temperature is held
constant, hence isotherm).
Freundlich
* Langmuir
* BET (Brunauer, Emmett, and
Teller)
* BJH (Barrett, Joyner, Halenda
There are several different
adsorption models (see below) which are based on
slightly different assumptions.
Langmuir equation
𝜃 =𝐾𝑃/(1 + 𝐾𝑃)
θ = number of sites covered with gas molecules
P = pressure, K = equilibrium constant
Only works well at low pressures and assumes
monolayer coverage.
BET Adsorption Isotherm
Multiple layer formation on a surface under high
pressures and low temperatures.
BJH Adsorption Isotherm
Multiple layer formation on a porous material
under high pressures and low temperatures. It
assumes the pores are cylindrical. Adsorption
into these materials occurs in two stages:
monolayer/multilayer adsorption on the walls,
filling by capillary condensation.
Experimental procedure (Surface area)
- Heat material under vacuum to remove
adsorbed species. - Cool the material under vacuum (77K).
- An adsorbent (N2
) is added in controlled
increments. - Pressure is allowed to equilibrate.
- Amount adsorbed is calculated.
- Surface area is determined by fitting into
different adsorption models
Experimental procedure (porosity)
As the pressure in the system increases, liquid
begins to condense in the pores. The pressure is
increased until the pores are saturated with
liquid, and then decreased until the liquid
evaporates. Adsorption, desorption and
hysteresis provide information about pore
volume, size and surface area
